1. Field of the Invention
This invention relates to a developer for a magnetic brush development in electrophotography.
2. Description of the Prior Art
According to electrophotographic processes, electrostatic latent images are produced on a photoconductive material by an electrostatic means and when a developer composed of toner particles and carrier particles is applied to the electrostatic latent images, toner particles separate from carrier particles to develop the electrostatic latent images. Such developing method is disclosed, for example, in U.S. Pat. No. 2,874,063 concerning a magnetic brush developing method. In such developing method, toner particles are held on the surface of carrier particles whose particle size is larger than that of toner particles by electrostatic force. The electrostatic force is caused by triboelectric charges of opposite polarity to each other produced by contact of toner particles with carrier particles. In such development, it is necessary that the toner particles have a proper polarity and electric charge quantity capable of being selectively attracted to electrostatic latent images when the developer composed of the toner particles and carrier particles contacts the electrostatic latent images.
In case of conventional dry developer, during development the carriers, the carrier and the toner, and the carrier, the toner and machine parts collide with each other many times. The resulting mechanical stress and heat make the toner particles adhere to the surface of the carrier particles and form a toner film thereon. Once such phenomenon as above occurs, the permanent film on the surface of the carrier particles accumulates as the development is repeated, and therefore, the ordinary triboelectric charging caused by rubbing between toner and carrier is partly replaced by triboelectric charging caused by rubbing between toner and toner. As a result, the triboelectric charge quantity changes with the lapse of time and the copied image quantity is lowered.
The toner film formation on the carrier surface may be prevented by coating the carrier surface with a low surface energy material as illustrated in U.S. Pat. Nos. 3,778,262 and 3,725,118.
Such coating with a low surface energy material is effective to prevent the toner film formation and prolong the life of developer, but the low surface energy material is insulative, and therefore edge effect occurs intensely, solid areas can not be uniformly developed, and bias potential is difficult to apply so that fog is liable to form on the background.